A Pleurosigma, a type of phytoplankton in the ocean. Phytoplankton generate more than half of the Earth's oxygen.
Harry Taylor/Nikon Small World
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Harry Taylor/Nikon Small World

Microscopic plants in the ocean, called phytoplankton, are among the most important creatures on Earth and produce half of the planet's oxygen. But they are in trouble. A new study finds that since 1950, the amount of phytoplankton in the ocean's surface waters — the basis of the ocean's food web — has declined by 40 percent.

Biologist Boris Worm is noted for his studies showing that the world's fisheries are in sharp decline. Most of that trend is due to overfishing, but it turns out that may not be the whole story. So the researcher at Dalhousie University in Nova Scotia has now turned his attention to the marine food chain and the ocean plant life that ultimately feeds almost everything else.

"The very fundamental question would be: Is the ocean getting more or less green? Is it increasing in plant life or decreasing?" Worm says.

This is a hard question to answer. Satellite studies show that the greenness of the ocean varies widely year to year, decade to decade. It would take many decades of data to see if there's a long-term trend.

Worm has now unearthed more than a century of data taken using an instrument called a Secchi disk, which measures the transparency of ocean water.

"The Secchi disk is a beauty because it is the simplest oceanographic instrument," Worm says. "It has also been in continuous use since it was invented in the late 1800s, and it hasn't changed since then."

Basically, oceanographers lower the white disk on a rope and note how deep it is when it disappears from view. Oceanographers have taken half a million measurements like this throughout the world's oceans, so Worm and his colleagues collected piles of that data and looked for trends.

"What we found was that phytoplankton was declining in 8 out of 10 large ocean regions," he says.

And the trend was pretty dramatic, averaging 1 percent per year, year after year, according to their study in this week's Nature.

Phytoplankton, like these Rhizosolenia setigera, form the base of the ocean's food web. A decline of phytoplankton will have an impact on the animals who rely on them for food.
Karl Bruun/Nostoca Algae Laboratory/Nikon Small World
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Phytoplankton, like these Rhizosolenia setigera, form the base of the ocean's food web. A decline of phytoplankton will have an impact on the animals who rely on them for food.

Karl Bruun/Nostoca Algae Laboratory/Nikon Small World

The cause is pretty clear: The declines are biggest where the ocean is warmest. As the surface of the ocean warms up, that hot water just sits there and prevents colder nutrient-rich waters from coming up from below, so the phytoplankton don't get fertilized.

And this turns out to be critical not just for plankton, but for the fish that feed on them.

"Areas that have a very high phytoplankton production, such as the Peruvian upwelling area, for example, have extremely high fisheries catches, and others with little phytoplankton production have low fisheries catches," Worm says.

And we know what happens when that Peruvian coastal area gets warmed up and its phytoplankton decline — that condition is called an El Nino. Michael Behrenfeld, a specialist in marine algae at Oregon State University, says El Ninos cause sharp declines in fish and in everything that depends on fish.

A large phytoplankton bloom in the northeast Atlantic Ocean, off the coast of France, Ireland and Great Britain, as seen from space.
NASA Earth Observatory Collection
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A large phytoplankton bloom in the northeast Atlantic Ocean, off the coast of France, Ireland and Great Britain, as seen from space.

NASA Earth Observatory Collection

"You see die-offs of marine mammals, die-offs of marine birds — so to me, that's just possibly the most tangible picture we can have of how important the productivity of these microscopic plants is to the welfare of the higher organisms that live and depend upon the ocean," Behrenfeld says.

Behrenfeld says El Nino is an analogy, not a prediction of what's going to happen. But there is no question the world's oceans have been getting warmer, and widely accepted forecasts say global warming will continue to make the oceans heat up.

Behrenfeld wasn't part of the study on phytoplankton, but he says the findings are quite consistent with similar studies, like his own, which have used satellites to measure declines in phytoplankton.

"None of them are perfect, but when you have multiple, independent ways of trying to answer the same question and they're all coming up with a similar result, that gives you power in terms of having confidence in the conclusions that you're drawing," he says.

As for Canadian biologist Boris Worm, when he's asked about what all this portends for the future of life in the ocean, he hesitates. "What I can say with certainty is it will be different."